1. Field of the Invention
The present invention generally relates to methods and apparatus for the in-situ detection of the loss or breakage of a workpiece during the chemical mechanical polishing (CMP) of the workpiece, and more particularly, to methods and apparatus for directing an ultrasonic signal onto the surface of a polishing pad or workpiece during polishing of the workpiece, and analyzing the reflection of the ultrasonic signal to detect real-time workpiece loss or breakage.
2. Description of the Related Art
The production of semiconductor devices begins with the creation of high quality semiconductor wafers. Because of the high precision required in the production of these semiconductor devices, an extremely flat surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures being created on the wafer surface. CMP is often used to remove material from the surface of the wafer or workpiece to provide a relatively flat surface.
Such polishing is well known in the art and generally includes placing one side of the workpiece in contact against a flat polishing surface, and moving the workpiece and the polishing surface relative to each other. A slury, including abrasive particles and/or chemicals that react with the material on the workpiece surface to dissolve the material, may also be placed in contact with the workpiece surface to assist removing a portion of the material. During the polishing or planarization process, the workpiece is typically held by a workpiece carrier and pressed against the polishing pad while the pad rotates. In addition, to improve the polishing effectiveness, the workpiece may also rotate and oscillate back and forth over the surface of the polishing pad.
During the CMP process, workpieces occasionally become dislodged from the workpiece carrier, or they may break during polishing. If a dislodged workpiece, a part of a broken workpiece, or other extraneous material is allowed to remain on the polishing table, it could contact other workpieces and/or workpiece carriers on the same polishing table and thereby damage or destroy all of the workpieces on the table. Accordingly, it is desirable to detect the presence of a broken or dislodged workpiece immediately and to terminate processing until the situation can be rectified. Typically, this requires a thorough cleaning and/or replacement of the polishing pad, so that workpiece fragments and other debris can be removed so that they do not damage other intact workpieces.
Presently known optical systems for detecting the loss of workpieces or for detecting broken workpieces are unsatisfactory in several regards. For example, currently known systems may be limited to operation with a small number of similarly colored polishing pads. Such known systems may be ineffective for detecting workpiece loss on a dark colored polishing pad or in an environment where the polishing pad may become discolored over time. Present optical systems may also be inadequate in CMP environments that employ a large amount of polishing slurry and/or polishing slurry having a variety of colors due to the effect of light scattering or transmission loss of the light signal. Furthermore, the presence of slurry, deionized water, and such staining CMP slurry compounds as potassium iodide and the like on the pad, and on the workpiece itself, tend to mask the reflected light signal, preventing the signal from being properly detected by the photo detector. Consequently, many presently known workpiece detection schemes often emit "false" readings whereupon machines are shut down and processing halted even though all workpieces remain intact within their respective carriers.
Therefore, a technique for detecting lost or dislodged workpieces on a CMP polishing pad is thus needed which overcomes the shortcomings of the prior art.